Project Abstract/Summary My long-term goal is to elucidate mechanisms for glial development. The focus of this application is to characterize local proliferation of astrocytes in the cerebral cortex and the mechanism underlying their interaction with blood vessels at the postnatal stage. During postnatal weeks 2 and 3, the density of blood vessels and the number of glial cells both increase by ~4 fold in the rodent brain. It is still unclear where the large number of astrocytes come from and how they interact with blood vessels. My preliminary data show that postnatal local proliferation of astrocytes is widely distributed in different layers of the cerebral cortex, and these astrocytes retain their endfeet (perivascular processes of astrocytes) with blood vessels while they enter mitotic stages. I thus hypothesize that local prolifaration of astrocytes is a main source of cortical astrocytes that contribute to endfoot formation in the postnatal brain. To address the hypothesis, I propose two specific aims: (1) Characterize local proliferation of astrocytes and their fate and function in the cerebral cortex; (2) Elucidate the mechanism underlying endfoot formation around blood vessels mediated by locally generated astrocytes. I have established two groups of transgenic mouse lines to sparsely label astrocytes via genetically encoded fluorescent markers with unprecedented resolution, along with methods I developed to label and record dividing cells. I am therefore uniquely poised to undertake this novel study. Research proposed in the K99 Mentored phase (year 1 and 2) is concentrated on the establishment of methods for retroviral labeling and imaging of glial proliferation in live animals, which will be carried out with the supervision of Dr. Lily Jan. The R00 Independent phase (years 3 to 5) will focus on characterizing the function of astrocytes generated via local proliferation, and the mechanism underlying their interaction with blood vessels. The proposed research is innovative because it will provide the first direct evidence of postnatal local proliferation of astrocytes in cerebral cortex and show how they integrate into glial network and form endfoot structures with blood vessels.